EFFECTS OF GREENHOUSE CO2 ENRICHMENT ON THE YIELD AND PHOTOSYNTHETIC PHYSIOLOGY OF TOMATO PLANTS

Abstract

Tomato crops were grown in greenhouses with and without CO2 enrichment to approximately 900 vpm. Plants grown under enhanced CO2 concentrations flowered earlier and produced 30% more marketable fruit than those grown in normal air. Measurements were conducted on CO2 and water vapor exchanges in apical and basal leaves under a range of irradiances and CO2 concentrations. Photosynthesis rates were higher in leaves from the enriched regime at test irradiances above 50 μE m−2 s−1 (400–700 nm). Increasing test CO2 concentration enlarged that difference, with the effect being most pronounced in apical leaves. Mesophyll resistance to CO2 assimilation was greater than stomatal resistance at all irradiances, and tended to be higher in basal leaves than in apical leaves of the CO2-enriched plants. Stomatal resistances were similar in apical and basal leaves from CO2-enriched plants. In unenriched plants, however, stomatal resistances were lower in apical than in basal leaves. CO2 compensation points were decreased in leaves developed under CO2 enrichment, but dark respiration rates were not significantly affected by growth regime. Behavioral indices of photosynthesis indicated that the efficiency of CO2 utilization was improved by growth in a CO2-enriched regime. Such fundamental changes in photosynthetic behavior suggest that the effects of CO2 enrichment on yield are not only due to increased growth in the presence of additional photosynthetic substrate. They also result from changes in the innate capacity of photosynthetic systems to utilize CO2.